1. Field of the Invention
This invention is related to injection molding apparatus controls and more specifically to a system for controlling part ejectors in injection molding apparatus.
2. State of the Prior Art
Injection molding, systems are used for molding plastic and some metal parts by forcing liquid or molten plastic materials or powdered metal in a plastic binder matrix into specially shaped cavities in molds where the plastic or plastic binder matrix is cooled and cured to make a solid part. For purposes of convenience, references herein to plastic and plastic injection molds are understood to also apply to powdered metal injection molding and other materials from which shaped parts are made by injection molding, even if they are not mentioned or described specifically.
A typical injection mold is made in two separable portions or mold halves that are configured to form a desired interior mold cavity or plurality of cavities when the two mold halves are mated or positioned together. Then, after liquid or molten plastic is injected into the mold to fill the interior mold cavity or cavities and allowed to cool or cure to harden into a hard plastic part or several parts, depending on the numbers of cavities, the two mold halves are separated to expose the hard plastic part or parts so that the part or parts can be removed from the interior mold cavity or cavities.
In many automated injection molding systems, ejector apparatus are provided to dislodge and push the hard plastic parts out of the mold cavities. A typical ejector apparatus includes one or more elongated ejector rods extending through a mold half into the cavity or cavities and an actuator connected to the rod or rods for sliding or stroking them longitudinally into the cavity or cavities to push the hard plastic part or parts out of the cavity or cavities. However, other kinds of ejector apparatus, such as robotic arms, scrapers, or other devices may also be used. Such ejectors are usually quite effective for dislodging and pushing hard plastic parts out of mold cavities, but they are not foolproof. It is not unusual for an occasional hard plastic part to stick or hang-up in a mold cavity in spite of an actuated ejector. Various methods have been used in efforts to prevent or at least minimize instances when hard plastic parts stick in mold cavities. For example, non-stick coatings, air knives, special mold designs, and other techniques have been tried and some used with various degrees of effectiveness to keep the hard plastic parts from sticking and to enhance their release from the mold halves. One quite common technique used alone or in conjunction with one or more of the techniques described above is to design and set the ejectors to actuate or stroke multiple times in rapid succession, such as four or five cycles each time a hard plastic part is to be removed, so that if a part sticks or is not removed from a mold cavity the first time it is pushed by an ejector, perhaps it can be dislodged by one or more subsequent hits or pushes from the ejectors. Such multiple ejector cycles are often effective to dislodge and clear the hard molded plastic parts from the molds. Disadvantages of multiple ejector cycling, however, include the additional time required for the multiple ejector cycling each time the mold is opened to eject a hardened plastic part before it is closed for injection of a subsequent part and the additional wear and tear on the ejector equipment and the molds occasioned by such multiple cycling. Over the course of days, weeks, and months of injection molding parts in repetitive, high volume production line operations, such additional time, wear, and tear can be significant production quantity and cost factors.
On the other hand, stuck or incompletely ejected hard plastic parts can also cause substantial damage to molds and lost production time. In most injection mold production lines, the injection molding machines operate automatically, once the desired mold is installed, in continuous repetitive cycles of closing the mold halves together, heating them, injecting liquid or molten plastic into the mold cavities, cooling to cure or harden the plastic in the mold into hard plastic parts, opening or separating the mold halves, ejecting the molded hard plastic parts, and closing the mold halves together again to mold another part or set of parts. Very high injection pressures are required to inject the liquid or molten plastic into the mold cavities to completely fill all portions of the cavities in a timely manner, and such high pressures tend to push the mold halves apart during injection of the plastic. To prevent such separation of the mold halves during plastic injection, most injection molding machines have very powerful mechanical or hydraulic rams to push and hold the mold halves together. If a hard plastic part from the previous cycle is not ejected and completely removed from between the mold halves, the powerful mechanical or hydraulic rams will try to close the mold halves onto the hard plastic part, which can and often does damage one or both of the mold halves. Molds are usually machined very precisely from stainless steel or other hard metal, so they are very expensive to replace, and the down-time required to change them is also costly in labor and lost production. It is also not unusual for some of the plastic in a mold cavity to break apart from the rest of the part being molded in the cavity and remain in the mold cavity when the rest of the molded part is ejected. Such remaining material will prevent proper filling and molding of subsequent parts in the cavity, thus causing the subsequent molded parts to be defective. In automated production lines, substantial numbers of such defective parts can be produced before someone detects them and shuts down the injection molding machine for correction of the problem.
To avoid such mold damage, down-time, and defective molded parts as described above, various technologies have also been developed and used to sense or determine whether the hard molded plastic parts have indeed been dislodged and completely ejected or removed from the molds before the mechanical or hydraulic rams are allowed to close. Such technologies have included light beam sensors, vision systems, air pressure sensors, vacuum sensors, and others. The U.S. Pat. No. 4,841,364 issued to Kosaka et al. is exemplary of a vision system in which video cameras take video images of the open mold halves for computerized comparison to video images of the empty mold halves stored in memory to detect any unremoved plastic parts or residual plastic material in the mold halves. The Bangerter et al. U.S. Pat. No. 4,603,329 shows an optoelectric sensor system for sensing presence or absence of the molded plastic parts, while the Mislan U.S. Pat. No. 3,303,537 uses infrared sensors to detect heat from any plastic that may be retained in the mold. The U.S. Pat. No. 4,236,181 uses a television camera to put video images of mold halves on a cathode ray tube (CRT) or television screen to detect images of any plastic parts in the mold that should not be there after ejection.
All or at least most of such part or plastic material detection systems provide some kind of interlock circuit connected or interfaced with the automatic cycling controls of automated injection molding machines to shut-down or otherwise prohibit the injection molding machines from closing the mold halves together if a plastic part or other material is still detected in one or both of the mold halves after the ejection portion of the molding cycle in order to avoid damage to the mold. Then the injection molding machine remains idle until an operator arrives to check the mold, clear any un-ejected plastic part or residual plastic material in the mold halves, and then disable the interlock to re-start the injection molding machines. Therefore, while such part sensing or detector systems avoid mold damage or defective molded parts by preventing the injection molding machines from closing the mold halves on un-ejected plastic parts or on uncleared plastic materials, down-time and loss of production can still be significant. To minimize such down time, it is a common practice to still use the multiple ejector cycles described above along with such part or plastic material sensor systems in order to avoid activation of the shut-down interlock at least for plastic parts or materials that may be dislodged by more than one hit or push by the ejector system. However, such multiple ejector cycles still have the disadvantages described above of taking more time between each opening and closing of the mold halves, and causes additional wear and tear on the ejector systems and molds each of which can accumulate to substantial lost production time over the course of days weeks, and months.